ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: The Infrared Excess of UV-Selected z = 2-10 Galaxies as a Function of UV-Continuum Slope and Stellar Mass
Carilli, Chris; Labbé, Ivo; Bouwens, Rychard J.; Daddi, Emanuele; van der Werf, Paul; Walter, Fabian; Oesch, Pascal; Bauer, Franz E.; Infante, Leopoldo; Aravena, Manuel; Chapman, Scott; Decarli, Roberto; Elbaz, David; Hodge, Jacqueline; da Cunha, Elisabete; Le Fevre, Olivier; Ota, Kazuaki; Cox, Pierre; Bertoldi, Frank; Weiss, Axel; Riechers, Dominik; Ivison, Rob J.; Wagg, Jeff; Smail, Ian R.; Magnelli, Benjamin; Wilkins, Steve; Karim, Alex; Gonzalez-Lopez, Jorge
Netherlands, Chile, Germany, United States, Australia, United Kingdom, Canada, France
Abstract
We make use of deep 1.2 mm continuum observations (12.7 μJy beam-1 rms) of a 1 arcmin2 region in the Hubble Ultra Deep Field to probe dust-enshrouded star formation from 330 Lyman-break galaxies spanning the redshift range z = 2-10 (to ∼2-3 M ⊙ yr-1 at 1σ over the entire range). Given the depth and area of ASPECS, we would expect to tentatively detect 35 galaxies, extrapolating the Meurer z ∼ 0 IRX-β relation to z ≥ 2 (assuming dust temperature T d ∼ 35 K). However, only six tentative detections are found at z ≳ 2 in ASPECS, with just three at >3σ. Subdividing our z = 2-10 galaxy samples according to stellar mass, UV luminosity, and UV-continuum slope and stacking the results, we find a significant detection only in the most massive (>109.75 M ⊙) subsample, with an infrared excess (IRX = L IR/L UV) consistent with previous z ∼ 2 results. However, the infrared excess we measure from our large selection of sub-L ∗ (<109.75 M ⊙) galaxies is {0.11}-0.42+0.32 ± 0.34 (bootstrap and formal uncertainties) and {0.14}-0.14+0.15 ± 0.18 at z = 2-3 and z = 4-10, respectively, lying below even an IRX-β relation for the Small Magellanic Cloud (95% confidence). These results demonstrate the relevance of stellar mass for predicting the IR luminosity of z ≳ 2 galaxies. We find that the evolution of the IRX-stellar mass relationship depends on the evolution of the dust temperature. If the dust temperature increases monotonically with redshift (\propto {(1+z)}0.32) such that T d ∼ 44-50 K at z ≥ 4, current results are suggestive of little evolution in this relationship to z ∼ 6. We use these results to revisit recent estimates of the z ≥ 3 star formation rate density.